The traditional direct-quadrature-zero (or dq0 or dqo) and alpha-beta (αβγ) transformation methods are usually adopted in control methods for power converters, synchronous machines and electronic devices in three-phase AC power systems. Reference can be made to the following patents:    [1] W. Said et al., “Active rectification for a variable-frequency synchronous generator,” Feb. 19, 2013, U.S. Pat. No. 8,378,644;    [2] T. Suzuki et al., “Converter and power converter that becomes it with the converter,” Dec. 30, 2008, U.S. Pat. No. 7,471,525;    [3] M. Kimura et al., “Electrical rotating machine control unit and power generation system,” Sep. 18, 2007, U.S. Pat. No. 7,271,500; and    [4] S. Bhowmik et al., “Performance optimization controller and control method for doubly-fed machines,” Aug. 25, 1998, U.S. Pat. No. 5,798,631.
The dq0-transformation can also be used in single phase systems in view of simplifying controller designs. Reference can be made to the following patent:    [5] R. Zhang, “Control of single-phase power converter in d-q rotating coordinates,” Sep. 16, 2003, U.S. Pat. No. 6,621,252.
The main advantage of these transformation methods is that they convert AC quantities into DC quantities, which can simplify the design of the control loop.
Recently, a new power electronic device known as an electric spring (ES), which is used in series connection with a non-critical load (such as thermal heaters) to form a smart load for active and reactive power compensation, has been invented. Reference can be made to the following articles and patents:    [6] S. Y. R. Hui et al., “Electric springs—a new smart grid technology,” IEEE Transactions on Smart Grid, vol. 3, no. 3, pp. 1552-1561, 2012;    [7] S. C. Tan et al., “General steady-state analysis and control principle of electric springs with active and reactive power compensations,” IEEE Transactions on Power Electronics, vol. 28, no. 8, pp. 3958-3969, 2013;    [8] S. Y. R. Hui et al., “Power control circuit and method for stabilizing a power supply,” Oct. 3, 2011, U.S. patent application Ser. No. 13/251,823;    [9] N. Chaudhuri et al., “Control methods for distributed power systems,” 2012, UK Patent Application No: 1206318.6;    [10] C. K. Lee et al., “Input AC voltage control bi-directional power converters,” May 31, 2013, U.S. patent application Ser. No. 13/907,350; and    [11] S. C. Tan et al., “Adaptive ac and/or dc power supply,” Jul. 9, 2013, U.S. Non-provision Patent No. 61/844,277.
The basic functionality of an ES is to regulate the electrical parameters of a power network such that its line voltage and frequency are within predetermined operating limits. However, existing controllers and control methods do not provide precise and flexible control of electrical parameters in a power network that employs ESs. This is of prime importance to the widespread application of ESs.
It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative.
Throughout this specification, a reference is initially cited with a number in square brackets and further citations of the same reference are made by citing said number in square brackets.